Non-aqueous secondary battery and a method of manufacturing graphite powder

ABSTRACT

Objects of the present invention is to provide a carbon material having a superior reversibility in lithium intercalation-deintercalation reaction, and a non-aqueous secondary battery using the carbon material as an active material for a negative electrode, which has a high energy density and an excellent rapid charging and discharging characteristics. 
     Graphite powder having a maximum particle diameter of less than 100 μm and an existing fraction of rhombohedral structure in the crystalline structure of less than 20% is used as an active material for the negative electrode of the non-aqueous secondary battery. The graphite powder can be obtained by pulverizing raw graphite with a jet mill, and subsequently treating the powder at a temperature equal to or higher than 900° C.

This application is a Divisional application of application Ser. No.08/630,501, filed Apr. 10, 1996 now U.S. Pat. No. 6,268,086.

BACKGROUND OF THE INVENTION

The present invention relates to a carbon material which intercalatesinto or deintercalates from lithium, and to a method for manufacturingthe same. In particular, the present invention relates to a lithiumsecondary battery, which uses carbon material as a negative electrodeactive material, having a high energy density and a long life. Thelithium battery is suitable for use in portable apparatus, electricautomobiles, power storage, etc.

The Lithium secondary battery using lithium metal for the negativeelectrode has some problems relating to safety. For example, lithiumeasily deposits like dendrite on the lithium metal negative electrodeduring repeated charging and discharging of the battery, and if thedendritic lithium grows to a positive electrode, an internal shortcircuit will be caused between the positive electrode and the negativeelectrode.

Therefore, a carbon material has been proposed as the negative electrodeactive material in place of lithium metal. Charge and dischargereactions involve lithium ion intercalation into the carbon material anddeintercalation from the carbon material, and so lithium is hardlydeposited like dendrite. As for the carbon material, graphite isdisclosed in JP-B-62-23433 (1987).

The graphite disclosed in JP-B-62-23433 (1987) forms an intercalationcompound with lithium, because of intercalation or deintercalation oflithium. Thus, graphite is used as a material for the negative electrodeof the lithium secondary battery. In order to use graphite as thenegative active material, it is necessary to pulverize the graphite toincrease the surface area of the active material, which constitutes acharge and discharge reaction field, so as to proceed the charging anddischarging reactions smoothly. Desirably, it is necessary to pulverizethe graphite to powder having a particle diameter equal to or less than100 μm. However, as it is apparent from a fact that the graphite is usedas a lubricating material, the graphite easily transfers its layers.Therefore, its crystal structure is changed by the pulverizing process,and formation of the lithium intercalated compound might be influencedby undesirable effects. Accordingly, the graphite after the pulverizingprocess has a great deal of crystalline structural defects. In a casewhen the above graphite is used as an active material for the negativeelectrode of the lithium secondary battery, a disadvantage is causedthat a large capacity can not be obtained. Furthermore, preferableperformances of rapid charge and discharge are not obtained because thelithium intercalation-deintercalation reaction is disturbed by the abovedefects.

SUMMARY OF THE INVENTION

(1) Objects of the Invention

The object of the present invention is to solve the above problemsinvolved in the prior arts, to disclose a carbon material having a largelithium intercalation-deintercalation capacity and a method formanufacturing the same, and to provide a non-aqueous secondary batterywhich has a large capacity and is superior in the rapid charging anddischarging characteristics using the above disclosed materials.

(2) Methods of Solving the Problems

The crystalline structure of the graphite powder relating to the presentinvention has a feature that an existing fraction of the rhombohedralstructure in the crystalline structure of the graphite is small (equalto or less than 20%). Another feature is that an existing fraction ofthe hexagonal structure is great (at least 80%). The above existingfractions of the rhombohedral structure and the hexagonal structure canbe determined by analyzing the intensity ratio of the peaks in the X-raydiffraction.

The graphite powder relating to the present invention is manufactured bya method comprising the steps of graphitizing treatment (heating atleast 2000° C.) of raw material such as oil cokes and coal cokes,pulverizing the graphitized raw material to powder, sieving the powderfor obtaining the maximum particle diameter equal to or less than 100μm, heating the powder at least 900° C. as a heat treatment, and furtherheating the powder at least 2700° C. for eliminating impurities such asSi. For instance, when the powder is heated at least 2700° C., Si, whichis a main component of impurities, can be reduced to less than 10 ppm.The heat treatment of the powder for eliminating impurities can beomitted depending on the content of the impurities in the raw material.In the pulverizing process, various conventional pulverizers can beused. However, a jet mill is preferable, because pulverization with thejet mill generates the minimum destruction of the graphite crystallinestructure in the raw material.

Furthermore, the graphite powder relating to the present invention canbe obtained by immersing into an acidic solution containing at least onecompound selected from a group consisted of sulfuric acid, nitric acid,perchloric acid, phosphoric acid, and fluoric acid as an immersingtreatment after pulverizing the raw graphite to obtain graphite powderhaving a particle diameter equal to or less than 100 μm, subsequentlywashing with water, neutralizing, and drying.

The non-aqueous secondary battery for achieving the object of thepresent invention can be manufactured by using the graphite powderrelating to the present invention as the negative electrode activematerial, and the positive electrode is desirably composed of a materialcomprising a compound expressed by a chemical formula of Li_(x)MO₂(where; X is in a range from zero to 1, and M is at least any one ofchemical elements selected from a group of Co, Ni, Mn and Fe), orLiMn₂O₄, that is a lithium transient metal complex oxide.

The active materials for the battery are generally used in a form ofpowder in order to proceed the charging and discharging reaction byincreasing the surface area of the active material, which is a reactionfield of the charging and discharging reaction. Therefore, the smallerthe particle size of the powder is, the more will performance of thebattery be improved. Furthermore, when the electrode is manufactured byapplying an agent mixed with the active material and a binding agent toa current collector, the particle diameter of the active material isdesirably equal to or less than 100 μm in view of applicability andmaintaining preciseness of thickness of the electrode.

As for the negative electrode active material for the lithium secondarybattery, natural graphite, artificial graphite, and others aredisclosed. However, the above described reason, it is necessary topulverize these materials. Therefore, in the pulverizing process,various graphite powder having a diameter equal to or less than 100 μmwere prepared with various pulverizing methods using a ball mill, a jetmill, a colloidal mill and other apparatus, for various time. And,lithium intercalation-deintercalation capacity of the various graphitepowder were determined for searching a superior material for thenegative electrode material of the lithium secondary battery.

However, the graphite powder obtained by the above method had thelithium intercalation-deintercalation amounts per weight in a range of200-250 mAh/g, and their capacities as the material for the negativeelectrode of the lithium secondary battery were not enough.

In order to investigate the reason for the small capacity, crystallinestructures of the above various graphite were determined by an X-raydiffraction method. FIG. 1 indicates an example of the results. Fourpeaks can be observed in a range of the diffraction angle (2θ, θ: Braggangle) from 40 degrees to 50 degrees in the X-ray diffraction pattern.The peaks at approximately 42.3 degrees and 44.4 degrees are diffractionpatterns of (100) plane and (101) plane of hexagonal structure of thegraphite, respectively. The peaks at approximately 43.3 degrees and 46.0degrees are diffraction patterns of (101) plane and (102) plane ofrhombohedral structure of the graphite, respectively. As explainedabove, it was apparent that there were two kinds of crystallinestructure in the pulverized graphite.

Further, the existing fraction (X) of the rhombohedral structure in thegraphite powder was calculated by the following equation (Equation 1)based on the data of the observed peak intensity (P₁) of the (100) planeof the hexagonal structure, the observed peak intensity (P₂) of the(101) plane of the rhombohedral structure, and a theoreticalrelationship of the intensity ratio in the X-ray pattern of graphite. Asthe result, it was revealed that the graphite having the rhombohedralstructure was contained by approximately 30% in all the graphitepulverized equal to or less than 100 μm in particle diameter.

X=3P ₂/(11P ₁+3P ₂)  (Equation 1)

Similarly, the existing fraction (X) of the rhombohedral structure ofthe graphite powder was verified by the relationship of the observedpeak intensity (P₁) of the (100) plane of the hexagonal structure, theobserved peak intensity (P₃) of the (102) plane of the rhombohedralstructure, and the theoretical relationship of the intensity ratio inthe X-ray pattern of graphite. In this case, the following equation 2was used instead of the equation 1. As the result, it was confirmed thatthe graphite having the rhombohedral structure was contained byapproximately 30% in all the graphite pulverized equal to or less than100 μm in particle diameter.

X=P ₃/(3P ₁ +P ₃)  (Equation 2)

The reason for existing the two kinds of crystalline structure isassumed that the graphite itself has a lubricating property, and theoriginal graphite having the hexagonal structure transforms to thegraphite having the rhombohedral structure by the pulverizing processwith strong shocks. The graphite powder having a few microns in particlediameter obtained by further continued pulverization had a significantlybroadened X-ray diffraction peak (P₄) at the (101) plane of thehexagonal structure, and it was revealed that the content of amorphouscarbon in the graphite was increased because the half band width of thepeak was increased. Accordingly, the reason for the small lithiumintercalation-deintercalation capacity of the conventional graphitepowder can be assumed that the crystalline structure of the graphite hastransformed to the rhombohedral structure and generated the amorphouscarbon, and proceeding of the lithium intercalation-deintercalationreaction is disturbed by the rhombohedral structure and the amorphouscarbon.

Analysis on the impurities of the graphite powder revealed that theimpurities such as Si, Fe, and others were contained more than 1000 ppm.Naturally, in addition to the impurities contained in the raw material,impurities from processing apparatus such as a ball mill, a jet mill,and the like can be mixed into the graphite at the pulverizing process.Therefore, the influence of the above impurities can be assumed asanother reason for the small capacity in addition to the above formationof the rhombohedral structure and amorphous carbon.

In accordance with the present invention, the graphite powder having aparticle diameter equal to or less than 100 μm, wherein the content ofthe above described rhombohedral structure is less than 30% and thecontent of the amorphous carbon is small, has been developed.Additionally, the content of Si in particular, which is the maincomponent of the impurities in the graphite powder, has been decreasedto equal to or less than 10 ppm. Therefore, extremely high purity is oneof the features of the graphite relating to the present invention. Theparticle diameter equal to or less than 100 μm is determined with anintention to use the graphite for the battery, as described previously.Therefore, when the graphite of the present invention is used for otherusages, the particle diameter of the graphite is not necessarilyrestricted to equal to or less than 100 μm.

Hereinafter, details of the graphite powder relating to the presentinvention, and the method for manufacturing the same is explained.

Two methods (manufacturing method 1 and manufacturing method 2) forobtaining the graphite having a small fraction of the rhombohedralstructure are disclosed.

(Manufacturing Method 1)

As for raw material (raw graphite) for the graphite powder of thepresent invention, both natural graphite and artificial graphite can beused. In particular, flaky natural graphite is preferable. Among theabove raw graphite, the one of which maximum diffraction peak in theX-ray diffraction pattern by the CuKα line is appeared at a diffractionangle (2θ, θ: Bragg angle) in a range form 26.2 degrees to 26.5 degrees,that is, an interval between two graphite layers is equal to or lessthan 0.34 nm, is desirable. Because, the graphite powder containing asmall amount of the rhombohedral structure can be obtained from the highcrystalline raw material.

As for the pulverizing apparatus for crushing the raw graphite to theparticle diameter equal to or less than 100 μm, a jet mill is desirable.The reason is that the amorphous carbon is generated less with the jetmill than the case when another pulverizing apparatus is used.

The pulverized raw graphite (raw powder) contains the graphite havingthe rhombohedral structure by approximately 30% as previously described.Then, in accordance with the present manufacturing method 1, theexisting fraction of the rhombohedral structure is made decrease by thefollowing heat treatment.

The heat treatment is performed at least 900° C. under an inert gasatmosphere. As for the inert gas, nitrogen gas, argon gas, and the likeis used. The inert gas atmosphere can also be maintained by covering theraw powder with cokes to seal it from the atmosphere.

The heat treatment is the most important process in the presentinvention for transforming the rhombohedral structure to the hexagonalstructure. It is necessary to perform the heat treatment afterpulverization of the raw graphite (more preferably, at the last stage ofthe graphite powder manufacturing process of the present invention).

If the heat treatment is performed before the pulverization of thegraphite and subsequently the graphite is pulverized, the graphitepowder containing the rhombohedral structure as small as possible, whichis the object of the present invention, can not be obtained. Thegraphite powder containing the rhombohedral structure graphite as smallas possible can be obtained only by the heat treatment after thepulverizing process (more preferably, at the last stage of the graphitepowder manufacturing process of the present invention) as the presentinvention.

The raw graphite powder contains Al, Ca, Fe, and particularly much ofSi, as impurities. The impurities can be eliminated by heating andsublimating the materials at least 2700° C. Therefore, the heatingtemperature in the heat treatment is preferably at least 2700° C. inorder to perform a purification treatment concurrently.

(Manufacturing Method 2)

The raw graphite and the pulverizing process is as same as the abovemanufacturing method 1.

The graphite powder of the present invention can be obtained by treatingthe graphite powder obtained by the pulverizing process with an acidicsolution containing at least one compound selected from a groupconsisted of sulfuric acid, nitric acid, perchloric acid, phosphoricacid, and fluoric acid, and subsequently washing with water,neutralizing, and drying. During the treatment, a compound is formedwith anions in the above acidic solution and the graphite, and therhombohedral structure graphite is eliminated by the formation of thecompound. The anions from the acidic solution in the compound areeliminated from the compound during the washing, the neutralizing, andthe drying, and the graphite powder relating to the present inventioncan be obtained.

The crystalline structure of the graphite powder of the presentinvention obtained by the above manufacturing methods 1 and 2 wasanalyzed by an X-ray diffraction. The ratio of the P₁ and P₂, (P₂/P₁),was less than 0.92, and the half band width of the P₄ was less 0.45degrees. The ratio of the P₁ and P₃, (P₃/P₁), was less than 0.75.

By substituting the above observed data for the equations 1 and 2, thefact that the existing fraction of the rhombohedral structure has beendecreased less than 20% and the existing fraction of the hexagonalstructure has been increased at least 80% was confirmed. Simultaneously,the content of Si was confirmed to be less than 10 ppm from the resultof impurity analysis.

Then, an electrode was prepared using the graphite powder of the presentinvention as an active material, and lithiumintercalation-deintercalation capacity was studied. As the result, thelithium intercalation-deintercalation capacity of the graphite powder ofthe present invention was 320-360 mAh/g per unit weight of the activematerial, and the capacity was significantly improved in comparison withthe capacity of the conventional graphite material (200-250 mAh/g).Furthermore, it was found that the preferable existing fraction of therhombohedral structure was equal to or less than 10%, because the lessthe existing fraction of the rhombohedral structure in the graphitepowder of the present invention is, the more will the capacity beincreased.

Accordingly, the rhombohedral structure is evidently a crystallinestructure which hardly intercalate or deintercalate lithium. Therefore,it is assumed that the high lithium intercalation-deintercalationcapacity of the graphite powder of the present invention is achieved byespecially decreasing the existing fraction of the rhombohedralstructure and increasing the existing fraction of the hexagonalstructure.

The feature of the lithium secondary battery of the present invention isin using the graphite powder of the present invention as the negativeactive material. The lithium secondary battery relating to the presentinvention has a large load capacity, and a high energy density can berealized.

As the result of an evaluation on the characteristics of the lithiumsecondary battery of the present invention, it was confirmed that thelithium secondary battery of the present invention had a superiorperformance in rapid charging and discharging characteristics, anddecreasing ratio of the capacity was improved at least 30% in comparisonwith the conventional lithium battery under a same rapid charging anddischarging condition. The reason for the improvement can be assumedthat the reversibility for the lithium intercalation-deintercalationreaction of the graphite of the present invention is improved incomparison with the conventional carbon material by decreasing theexisting fraction of the rhombohedral structure and eliminating theinfluence of the impurities such as Si.

As the positive active material for the lithium secondary battery of thepresent invention, materials such as Li_(x)CoO₂, Li_(x)NiO₂,Li_(x)Mn₂O₄, (where, X is in a range 0-1) and the like are desirablebecause a high discharge voltage of at least 3.5 V can be obtained, andthe reversibility of the charge and discharge of the positive electrodeitself is superior.

As for the electrolytic solution, a mixed solvent composed of ethylenecarbonate mixed with any one selected from a group consisted ofdimethoxyethane, diethylcarbonate, dimethylcarbonate,methylethylcarbonate, γ-butylolactone, methyl propionate, and ethylpropionate, and at least one of electrolytes selected from a groupconsisted of salts containing lithium such as LiClO₄, LiPF₆, LiBF₄,LiCF₃SO₃, and the like are used. It is desirable to adjust the lithiumconcentration in a range 0.5-2 mol/l, because, an electric conductivityof the electrolytic solution is favorably large.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 indicates an X-ray diffraction pattern of the conventionalgraphite,

FIG. 2 indicates an X-ray diffraction pattern of the graphite powderrelating to the embodiment 1 of the present invention (heat treatmenttemperature: 900° C.),

FIG. 3 indicates an X-ray diffraction pattern of the graphite powderrelating to the embodiment 1 of the present invention (heat treatmenttemperature: 2850° C.),

FIG. 4 indicates an X-ray diffraction pattern of the graphite powderprepared in the comparative example 1,

FIG. 5 indicates an X-ray diffraction pattern of the graphite powderrelating to the embodiment 2 of the present invention,

FIG. 6 indicates a schematic cross section of the battery used in theembodiment 3 and the comparative example 2,

FIG. 7 is a graph indicating a relationship between the electrodepotential and the lithium intercalation-deintercalation capacity,

FIG. 8 is a partial cross section of the lithium secondary batteryprepared in the embodiment 5 of the present invention,

FIG. 9 is a graph indicating a relationship between the dischargecapacity and the number of repeating the charge and the dischargecycles,

FIG. 10 is a graph indicating a relationship between the dischargecapacity and the charging and discharging current,

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to drawings, embodiments of the present invention areexplained hereinafter.

Embodiment 1

Flaky natural graphite which was produced from Madagascar was used asthe raw material, and the raw material was pulverized to be powder, ofwhich particle diameter was equal to or less than 46 μm, by a jet mill.The powder was sieved to obtain raw material powder. The averagediameter of the raw material powder was 8.0 μm. Subsequently, the rawmaterial powder was processed with a heat treatment by heating at 900°C. or 2850° C. for ten days under a nitrogen atmosphere, and thegraphite powder of the present invention was obtained.

The crystalline structures of the graphite powder of the presentinvention and the raw material powder were analyzed by an X-raydiffraction method using an apparatus RU-200 made by Rigaku Denki, andthe impurity content was analyzed by an inductively coupled plasmaspectrometry (ICP) using an apparatus P-5200 made by Hitachi.

The X-ray diffraction patterns of the graphite powder of the presentinvention, which have been observed under a condition of X-ray tubevoltage; 40 kV, X-ray tube current; 150 mA, and X-ray source; CuKα line,are shown in FIGS. 2 and 3. FIG. 2 is the pattern obtained by the heattreatment at 900° C., and FIG. 3 is the pattern obtained by the heattreatment at 2850° C. The X-ray diffraction patterns of the graphitepowder of the present invention in both FIG. 2 and FIG. 3 indicate thatthe peaks at diffraction angles of 43.3 degrees and 46.0 degrees, bothof which belong with the rhombohedral structure, are decreased by eitherof the above heat treatments.

The amount of Si contained in the graphite powder of the presentinvention as an impurity was 1140 ppm when the heating temperature was900° C., and 27 ppm when the heating temperature was 2850° C. Therefore,it is revealed that a highly purified graphite powder, of which Si iseliminated, can be obtained by heat treatment at a high temperature atleast 2700° C., by which Si can be eliminated.

COMPARATIVE EXAMPLE 1

In order to compare with the embodiment of the present invention, thenon-pulverized raw graphite was heated at 2850° C., and subsequentlypulverized to obtain the graphite powder. The X-ray pattern of thegraphite powder obtained by the above process is shown in FIG. 4. It isapparent from FIG. 4 that the peaks at diffraction angles of 43.3degrees and 46.0 degrees, both of which belong with the rhombohedralstructure, are not decreased. That means, the rhombohedral structure cannot be eliminated by the above process.

Embodiment 2

In accordance with the embodiment 2, the raw graphite was pulverized bya jet mill to less than 100 μm in particle diameter. Then, the graphitepowder was immersed into a mixed acid of sulfuric acid and nitric acidfor a whole day. Subsequently, washing with distilled water andneutralization with a dilute aqueous solution of sodium hydroxide wereperformed. The graphite powder obtained by the above process was driedat 120° C. to obtain the graphite powder of the present invention. TheX-ray pattern of the graphite powder obtained by the above process isshown in FIG. 5. The peaks at diffraction angles of 43.3 degrees and46.0 degrees, both of which belong with the rhombohedral structure, aredecreased. Accordingly, it was found that the rhombohedral structure waseliminated by the above process.

Embodiment 3

In accordance with the embodiment 3, a carbon electrode was preparedusing the graphite powder of the present invention as an electrodeactive material, and the lithium intercalation-deintercalation capacity,in other words, a load capacity of the negative electrode in the lithiumsecondary battery was studied with the electrode.

Mixed agents slurry were prepared by mixing 90% by weight in total solidof the graphite powder of the present invention prepared in theembodiment 1, 10% by weight of polyvinylidene fluoride (PVDF) as abinder, and N-methyl-2-pyrolidone of which heating temperature were 900°C. and 2850° C., respectively. The mixed agents slurry was applied on aplane of a sheet of copper foil of 10 μm thick, and dried in vacuum at120° C. for one hour. After the vacuum drying, an electrode wasfabricated by roller pressing, of which thickness was in a range 85-90μm. The average amount of the applied mixed agents per unit area was 10mg/cm². The electrode was prepared by cutting the copper foil appliedwith the mixed agents into a sheet of 10 mm×10 mm.

FIG. 6 is a schematic cross section of a battery used for studying thelithium intercalation-deintercalation capacity of the present electrode.The battery has a structure, wherein a working electrode currentcollector 30, the electrode of the present invention 31, which is aworking electrode, a separator 32, a lithium metal 33, which is acounter electrode, a counter electrode current collector 34 are piledand inserted into a battery vessel 35, and a battery lid 36 is screwedfor fixing. A reference electrode made of lithium metal 37 is attachedto the battery. As for the electrolytic solution, a mixed solvent ofethylene carbonate and diethylcarbonate by 1:1 in volume and lithiumhexafluorophosphate were used with lithium concentration of 1 mol/l.

The intercalation-deintercalation of lithium was repeated by applying aconstant current between the working electrode and the counterelectrode, and the capacity was determined. The terminated potentials ofthe intercalation and the deintercalation of the working electrode wereset as 0 V and 0.5 V, respectively.

COMPARATIVE EXAMPLE 2

In order to compare with the embodiment of the present invention, acarbon electrode was prepared with the graphite powder obtained in thecomparative example 1 by the same method as the embodiment 3, and theload capacity (the amount of lithium intercalation-deintercalation) wasdetermined. The same study was performed on the electrode prepared withthe conventional graphite powder (the same powder as the raw powder inthe embodiment 1).

A result of comparison on the lithium intercalation-deintercalationbehavior of the electrode in the embodiment 3 (the present invention)with the electrode in the comparative example 2 (prior art) and theelectrode prepared with the conventional graphite powder is explainedhereinafter. FIG. 7 is a graph indicating a relationship between thelithium intercalation-deintercalation capacity and an electrodepotential at the fifth cycle, wherein the capacity becomes stable, afterrepeating the intercalation-deintercalation of lithium. In FIG. 7, thecurve 40 indicates the potential variation of the electrode preparedwith the graphite powder, of which heating temperature at the heattreatment was 900° C., in the embodiment 3. The curve 41 indicates thepotential variation of the electrode prepared with the graphite powder,of which heating temperature at the heat treatment is 2850° C., in theembodiment 3. The curve 42 indicates the potential variation of theelectrode prepared with the conventional graphite powder, and the curve43 indicates the potential variation of the electrode prepared with thegraphite powder which has been prepared in the comparative example 1 bythe reversely ordered processes. The intercalation capacity and thedeintercalation capacity for lithium in both the cases of using theconventional graphite in the comparative example 2 (the curve 42) andthe graphite in the comparative example 1 (the curve 43) were less than250 mAh/g per unit weight of the active materials. On the contrary, inthe case of the embodiment 3 (the curves 40, 41), wherein the graphitepowder prepared in the embodiment 1 was used as the active material,both the intercalation capacity and the deintercalation capacity forlithium were more than 300 mAh/g per unit weight of the activematerials. That means, a large load capacity was obtained by using thegraphite powder having a small existing fraction of the rhombohedralstructure relating to the present invention. Furthermore, the case (thecurve 41) using the graphite powder highly purified by heating up to2850° C. indicates the largest values in both the intercalation capacityand the deintercalation capacity for lithium in FIG. 7.

Embodiment 4

The embodiment 4 was performed in order to confirm the influence oftreating time in the heat treatment of the present invention. In theembodiment 4, the graphite powder of the present invention was obtainedin accordance with the substantially same manner as the embodiment 1(under a nitrogen atmosphere, the raw powder was heated at 2850° C.).However, the treating time of the heat treatment was varied in a rangefrom 0 hours to 30 days.

The existing fraction of the rhombohedral structure was determined fromthe peak intensity in X-ray diffraction patterns. Furthermore, as sameas the embodiment 3, the electrodes were prepared with the obtainedgraphite powders, and the intercalation-deintercalation reactions oflithium were repeatedly performed. The result on the lithiumintercalation-deintercalation capacity at the fifth cycle is shown inTable 1.

TABLE 1 The existing fraction of Lithium Lithium the intercalationdeintercalation rhombohedral capacity capacity Heating time structure(%) (mAh/g) (mAh/g) 0 hours 27.3 249 235 4 hours 18.2 332 320 10 hours14.6 345 325 1 day 13.8 343 334 3 days 11.3 355 338 5 days 9.7 368 35110 days 7.1 365 360 30 days 3.9 366 361

In accordance with the above result, it is apparent that the smaller theexisting fraction of the rhombohedral structure is, the more will thelithium intercalation-deintercalation capacity be increased. Inparticular, the existing fraction equal to or less than 10% isdesirable.

Embodiment 5

The present embodiment uses a cylindrical lithium secondary battery. Afundamental structure of the secondary battery is shown in FIG. 8. InFIG. 8, the member assigned with the numeral mark 50 is a positiveelectrode. Similarly, a negative electrode 51, a separator 52, apositive electrode tab 53, a negative electrode tab 54, a positiveelectrode lid 55, a battery vessel 56, and a gasket 57 are shown.

The lithium secondary battery shown in FIG. 8 was prepared by thefollowing steps. Mixed positive electrode agents slurry was prepared bymixing 88% by weight in total solid of LiCoO₂ as an active material forthe positive electrode, 7% by weight of acetylene black as a conductiveagent, 5% by weight of polyvinylidene fluoride (PVDF) as a binder, andN-methyl-2-pyrolidone.

Similarly, mixed negative electrode agents slurry was prepared by mixing90% by weight in total solid of the graphite powder of the presentinvention as an active material for the negative electrode, 10% byweight of polyvinylidene fluoride (PVDF) as a binder, andN-methyl-2-pyrolidone.

The mixed positive electrode agents slurry was applied onto both planesof a sheet of aluminum foil of 25 μm thick, and dried in vacuum at 120°C. for one hour. After the vacuum drying, an electrode of 195 μm thickwas fabricated by roller pressing. The average amount of the appliedmixed agents per unit area was 55 mg/cm². The positive electrode wasprepared by cutting the aluminum foil applied with the mixed agents intoa sheet of 40 mm in width and 285 mm in length. However, portions of 10mm in length from both ends of the positive electrode were not appliedwith the mixed agents for the positive electrode, the aluminum foil wasbared, and one of the bared portion was welded to the positive electrodetab by ultrasonic bonding.

The mixed negative electrode agents slurry was applied onto both planesof a sheet of copper foil of 10 μm thick, and dried in vacuum at 120° C.for one hour. After the vacuum drying, an electrode of 175 μm thick wasfabricated by roller pressing. The average amount of the applied mixedagents per unit area was 25 mg/cm². The negative electrode was preparedby cutting the copper foil applied with the mixed agents into a sheet of40 mm in width and 290 mm in length. However, as same as the positiveelectrode, portions of 10 mm in length from both ends of the negativeelectrode were not applied with the mixed agents for the negativeelectrode, the copper foil was bared, and one of the bared portion waswelded to the negative electrode tab by ultrasonic bonding.

A fine pored film made of polypropylene of 25 μm thick and 44 mm inwidth was used as a separator. The positive electrode, the separator,the negative, and the separator were piled in the order described above,and the pile was rolled to form a bundle of the electrodes. The bundlewas contained in a battery vessel, the negative electrode tab was weldedto the bottom of the battery vessel, and a drawn portion for caulkingthe positive electrode lid was fabricated. An electrolytic solutionprepared by adding lithium hexafluorophosphate by 1 mol/l into a mixedsolvent containing ethylene carbonate and diethylcarbonate by 1:1 involume was filled the battery vessel, the positive electrode tab waswelded to the positive electrode lid, and the positive electrode lid wascaulked to the battery vessel to form the battery.

Using the battery which had been prepared by the above steps, the chargeand discharge were repeated under a condition that the charging anddischarging current was 300 mA, and respective of the terminatedpotentials of the charge and the discharge was 4.2 V and 2.8 V.Furthermore, the charging and the discharging current was varied in arange from 300 mA to 900 mA, and the rapid charge and rapid dischargewere performed.

COMPARATIVE EXAMPLE 3

In order to compare with the present invention, a lithium secondarybattery was manufactured by the method as same as the embodiment 5 usingthe conventional graphite powder (the raw powder for the graphite powderof the present invention), and the battery characteristics wasdetermined as same as the embodiment 5.

The result of comparison on the characteristics of the lithium secondarybattery of the embodiment 5 (the present invention) and the comparativeexample 3 (prior art) is explained hereinafter.

FIG. 9 indicates variation in discharge capacity of the lithiumsecondary battery when the charge and discharge of the battery wererepeated. The curve 60 indicates the discharge capacity of theembodiment 5. The curve 61 indicates the discharge capacity of thecomparative example 3. In the embodiment 5, the maximum dischargecapacity was 683 mAh, and a ratio in the discharge capacity after 200cycles to the maximum capacity was 86%. While, in the comparativeexample 3, the maximum discharge capacity was 492 mAh, and a ratio inthe discharge capacity after 200 cycles to the maximum capacity was 63%.

FIG. 10 indicates a relationship between the charging and dischargingcurrent and the discharge capacity when the rapid charge and rapiddischarge were performed. The curve 70 indicates the discharge capacityof the embodiment 5. The curve 71 indicates the discharge capacity ofthe comparative example 3. With the charging and discharging current of900 mA, the discharge capacity of the embodiment 5 was 573 mAh, whilethe discharge capacity of the comparative example 3 was 256 mAh. Theratio of decreasing the discharge capacity in the respective of thepresent cases to the discharge capacity in the case of charging anddischarging current of 300 mAh/g were 16% and 48%, respectively.Therefore, in accordance with using the graphite powder of the presentinvention as the active material for the negative electrode, thedecreasing ratio of the capacity was improved by at least 30%, and itbecame apparent that the lithium secondary battery relating to thepresent invention had an excellent characteristics in rapid charge anddischarge.

Embodiment 6

Mixed positive electrode agents slurry was prepared using LiMn₂O₄ as apositive electrode active material, and the positive electrode wasprepared by applying the mixed positive electrode agents slurry ontoboth planes of a sheet of aluminum foil by as same as the embodiment 5.The average amount of the applied mixed agents per unit area was 65mg/cm², and the electrode thickness after fabrication by roller pressingwas 230 μm. The positive electrode was prepared by cutting the aluminumfoil applied with the mixed agents into a sheet of 40 mm in width and240 mm in length. However, portions of 10 mm in length from both ends ofthe positive electrode were not applied with the mixed agents for thepositive electrode. The negative electrode was as same as the negativeelectrode prepared in the embodiment 5. Then, the lithium secondarybattery of the present embodiment was prepared by the same method as theembodiment 5 such as forming an electrodes bundle, inserting theelectrodes bundle into a vessel, welding bottom of the vessel, adding anelectrolytic solution, caulking a positive electrode lid, and others.

Using the battery, charge and discharge were repeated under a conditionof charging and discharging current of 300 mA, and the terminatedpotential of the charge and discharge of 4.2 V and 2.8 V, respectively.As the result, the maximum discharge capacity was 581 mAh, and a ratioin the discharge capacity after repeating the charging and dischargingreactions 200 cycles to the maximum discharge capacity was 84%. Theabove result indicates that the charging and discharging characteristicsof the present embodiment is superior to the comparative example 3.

A lithium secondary battery which has a high energy density and anexcellent charging and discharging characteristics can be obtained byusing the graphite powder, which is superior in reversibility of theintercalation-deintercalation reaction of lithium, of which maximumparticle size is less than 100 μm, wherein the existing fraction of therhombohedral structure in the crystalline structure is less than 20%, asthe active material for the negative electrode of the battery.

What is claimed is:
 1. A method of manufacturing graphite powder adaptedto be an active material of a lithium battery negative electrode,comprising the steps of: pulverizing raw graphite, to produce pulverizedgraphite having a crystalline structure which is at least 30% by weightrhombohedral crystalline structure and at most 70% by weight hexagonalcrystalline structure; sieving said pulverized graphite to recovergraphite powder having a maximum particle diameter of 100 μm; heatingsaid graphite powder as a heat treatment to transform the crystallinestructure to at least 80% by weight hexagonal crystalline structure andat most 20% by weight rhombohedral crystalline structure; and furtherheating said graphite powder, at a higher temperature than said heattreatment to transform the crystalline structure, to eliminateimpurities.
 2. A method of manufacturing graphite powder as claimed inclaim 1, wherein the temperature of said heat treatment to transformcrystalline structure to hexagonal structure is at least 900° C.
 3. Amethod of manufacturing graphite powder as claimed in claim 1, whereinthe temperature of said heat treatment to eliminate impurities is atleast 2700° C.
 4. A method of manufacturing graphite powder as claimedin claim 2, wherein the temperature of said heat treatment to transformcrystalline structure to hexagonal structure is in a range of 900° C. to1100°.
 5. A method of manufacturing graphite powder as claimed in claim3, wherein the temperature of said heat treatment to eliminateimpurities is in a range of 2700° C. to 2900° C.
 6. A method ofmanufacturing graphite powder as claimed in claim 1, wherein saidgraphite powder is in powder form during said heating as a heattreatment to transform the crystalline structure.
 7. A method ofmanufacturing graphite powder adapted to be an active material of alithium battery negative electrode, comprising the steps of: pulverizingraw graphite, to produce pulverized graphite having a crystallinestructure which is at least 30% by weight rhombohedral crystallinestructure and at most 70% by weight hexagonal crystalline structure;sieving said pulverized graphite to obtain graphite powder having amaximum particle diameter of 100 μm; and either (a) heating saidgraphite powder as a heat treatment to transform the crystallinestructure of said graphite powder to at least 80% by weight hexagonalcrystalline structure and at most 20% by weight rhombohedral crystallinestructure, and further heating said graphite powder, at a highertemperature than said heat treatment to transform the crystallinestructure, to eliminate impurities; or (b) immersing said graphitepowder, into an acidic solution as an immersing treatment, washing withwater, neutralizing and drying, to transform the crystalline structureof said graphite powder to at least 80% by weight hexagonal crystallinestructure and at most 20% by weight rhombohedral crystalline structure.8. A method of manufacturing graphite powder as claimed in claim 7,wherein said graphite powder is in powder form during said heating as aheat treatment to transform the crystalline structure, or during saidimmersing.
 9. A method of manufacturing graphite powder adapted to be anactive material of a lithium battery negative electrode, comprising thesteps of: providing graphite powder having a particle size equal to orsmaller than 100 μm, and having a crystalline structure which is atleast 30% by weight rhombohedral crystalline structure and at most 70%by weight hexagonal crystalline structure; and heating said graphitepowder as a heat treatment, or immersing said graphite powder into anacidic solution as an immersing treatment, to form treated graphitepowder, such that the treated graphite powder has a fraction of ahexagonal structure of at least 80% by weight and a fraction of arhombohedral structure of at most 20% by weight.
 10. A method ofmanufacturing graphite powder as claimed in claim 9, wherein thegraphite powder has a fraction of a hexagonal structure of at least 90%by weight.
 11. A method of manufacturing graphite powder as claimed inclaim 10, wherein the graphite powder has a fraction of a rhombohedralstructure of at most 10% by weight.
 12. A method of manufacturinggraphite powder as claimed in claim 9, wherein said graphite powder isin powder form during said heating as a heat treatment, or during saidimmersing, to form the treated graphite powder.
 13. A method ofmanufacturing graphite powder adapted to be an active material of alithium battery negative electrode, comprising the steps of: providinggraphite powder having a particle size equal to or smaller than 100 μm,and having a crystalline structure which is at least 30% by weightrhombohedral crystalline structure and at most 70% by weight hexagonalcrystalline structure; and treating the graphite powder such that thetreated graphite powder has a fraction of a hexagonal structure of atleast 80% by weight and a rhombohedral structure of at most 20% byweight.
 14. A method of manufacturing graphite powder as claimed inclaim 13, wherein said graphite powder is in powder form during saidtreating.
 15. A method of manufacturing graphite powder as claimed inclaim 9, wherein the graphite powder is provided by pulverizing rawgraphite; and wherein the graphite powder is heated to form treatedgraphite powder, and after said heating the graphite powder is furtherheat-treated, at a higher temperature than the temperature of saidheating, to eliminate impurities.